Just got another dent this past week 79 f150 4x4 lwb 351m c6 NP205. Its all stock on 31's. I wanted the truck to build a good driver out of and thought it had 3.50 gears in it when I bought it.(read the tag wrong it has 4.10) Anyways My highboy has 4.10 on 33's with 360 NP203, c6 and it gets 10mpg going 55mph but sound like its revving to hell at 2600rpm. I wanted the f150 to be more of a highway cruiser. I got a tach to put in the f150 to see if its rpms are as high as the highboys on the highway just need to get it on the truck. What are your highway rpms? Would it be worth m y time to swap gears to 3.50? What kind of mpg gains could I get?

 

This calculator may help.

http://www.ringpinion.com/Calculators/Calc_RPM.aspx

I run a 79 f250 460 with stock 29s and it's 2500 rpm at 60mph with a 3.54 rear end gear. If your gonna have it a while, the gear swap would be worth it. The gear vendors unit or trans swap don't seem like they pay themselves back. These trucks were never made for super highway cruising or great fuel economy. If you do make the swap, don't go any further upstream than the diffs. Waste of money and time. Changing 4.10s to 3.50s is the equivalent of adding an over drive to the 4.10 truck but you loose torque to the wheels for towing and starting out.

Tough to quantify mpg gain but you would knock engine rpm down by about 400 which would help with engine life.